1. Field of the Invention
The present invention relates to servo-systems used in self-balancing instruments, and more particularly to servo-systems using an ac servomotor. They are of a type which may be used in feedback thermocouple or resistance thermometers.
2. Description of the Prior Art
In servo-systems of the type using an ac servomotor a dc deviation signal between the input signal and the compared signal is converted into an ac signal by a chopper and then amplified by an ac amplifier; the ac servomotor is driven by the amplified signal, and the brush of the slide resistor of the compared value generator is displaced, thus causing the dc deviation signal to be zero-balanced.
In such systems, the chopper is coupled to the ac amplifier through a coupling capacitor. When a capacitor is associated with the servo loop, however, the servo loop is liable to become unstable due to a phase shift by the capacitor.
In recent servo-systems such as that disclosed in U.S. Pat. No. 3,866,103 to Maezawa, et al., an increasing number of linear IC's have come to be used as amplifiers. The linear IC is a direct-coupling amplifier; hence it is operated in such manner that a negative feedback is effected in dc and low frequency ranges to lower its gain and thereby to operate the amplifier for normal ac amplifying functions. This necessitates a high frequency bypass filter for the feedback circuit of the linear IC amplifier, with the result that circuit construction becomes intricate and the servo loop tends to operate unstably due to a phase shift by the capacitor.
Furthermore, in this type of servo-system, the input signal variable range is wide; for example, in a self-balancing instrument, the input span ranges from a minimum of 5 mV to a maximum of 25 V, that is, the maximum span is often greater than the minimum span by a factor of 10.sup.3.
In prior art circuits a voltage derived from the slide resistor through a span adjusting voltage divider is used as the compared value, thereby making it possible for the input span to correspond to 0 to 100% points on the slide resistor.
In a practical circuit, a change in the ratio of the voltage divider for span adjustment will vary the loop gain of the servo loop, which variation may be great especially in a self-balancing instrument. However, it is desirable that the loop gain of a servo loop be constant, because the nonlinearity and transfer function of the servo loop are in a dynamic system. In span adjustment by prior art, therefore, the gain of the servo amplifier has had to be manually adjusted with care not to affect the loop gain at the sacrifice of extra adjusting efforts and increase in the number of circuit elements.
This type of servo-system is often used for a thermoelectric thermometer using a thermocouple and for a resistance thermometer using a resistance bulb. In applications to the former, it is desirable that the thermocouple output be automatically compensated for by an electrical signal which corresponds to the temperature at the cold junction because the thermocouple generates a thermal emf corresponding to the difference between the temperature detected at the hot junction and that at the cold junction. In applications to the latter, changes in the resistance of the bulb should be as linear as possible although such resistance changes tend to be nonlinear against temperatures detected at the hot junction. It is also desirable that the resistance thermometer be free of the resistance of the lead wire from the resistance bulb because the lead wire resistance often serves as a source of error in measurements.